This code adds a "discount" to catalog ordering args. It makes it possible for a user to order products on WooCommerce category pages and the main shop page by the product's discount amount. It also adds a "'Sort by discount" to sorting menus on shop page and category pages. Finally it displays the percentage you save on a discounted item. This …

product_discount_sorting_and_displaying.php

<?php

/**
 * Add "discount" to catalog ordering args.
 *
 * This makes it possible for a user to order products on category pages and the main shop
 * page by the product's discount amount.
 *
 * @see wc_clean()
 * @link http://docs.woothemes.com/document/custom-sorting-options-ascdesc/
 *
 * @param array $args Ordering arguments.
 *
 * @return array Updated $args array.
 */
add_filter( 'woocommerce_get_catalog_ordering_args', 'mycode_add_salediscount_to_woocommerce_catalog_ordering_args' );
function mycode_add_salediscount_to_woocommerce_catalog_ordering_args( $args ) {

	$orderby_value = isset( $_GET['orderby'] ) ?
		wc_clean( $_GET['orderby'] ) :
		apply_filters( 'woocommerce_default_catalog_orderby', get_option( 'woocommerce_default_catalog_orderby' ) );

	if ( 'discount' == $orderby_value ) {
		$args['orderby']  = 'meta_value_num';
		$args['order']    = 'DESC';
		$args['meta_key'] = '_dfrps_salediscount';
	}

	return $args;
}

/**
 * Adds "'Sort by discount" to sorting menus on shop page and category pages.
 *
 * @link https://docs.woocommerce.com/document/custom-sorting-options-ascdesc/
 *
 * @param array $sortby Array of keys to sort by.
 *
 * @return array Updated $sortby array.
 */
add_filter( 'woocommerce_default_catalog_orderby_options', 'mycode_woocommerce_add_salediscount_to_catalog_orderby' );
add_filter( 'woocommerce_catalog_orderby', 'mycode_woocommerce_add_salediscount_to_catalog_orderby' );
function mycode_woocommerce_add_salediscount_to_catalog_orderby( $sortby ) {
	$sortby['discount'] = 'Sort by discount';

	return $sortby;
}

/**
 * Displays the percentage you save on a discounted item.
 *
 * This will echo HTML like "Save 34%" near the price of a product which is on sale.
 *
 * @global WC_Product $product
 */
add_action( 'woocommerce_after_shop_loop_item', 'mycode_show_discount_in_product_lists' ); // Product Loop
add_action( 'woocommerce_single_product_summary', 'mycode_show_discount_in_product_lists' ); // Product Page
function mycode_show_discount_in_product_lists() {

	global $product;

	$salediscount = get_post_meta( $product->id, '_dfrps_salediscount', true );

	if ( $salediscount > 0 ) {
		echo '<div class="dfrps_salediscount">';
		echo 'Save ' . $salediscount . '%';
		echo '</div>';
	}
}